KU Leuven: Nearest ‘galaxy with a black hole’ appears to contain no black hole

In 2020, a team led by astronomers from the European Southern Observatory (ESO) reported a black hole in the star system HR 6819. Just 1,000 light-years away, it would be the closest black hole. However, their discovery was disputed by other researchers, including an international research team from KU Leuven. In an article published today in Astronomy & Astrophysics, the two teams now jointly conclude that HR 6819 does not contain a black hole: it is a binary star that is in a rare and short-lived phase of evolution.

The original research article on HR 6819 received a lot of attention from both the press and scientists. Chilean ESO astronomer Thomas Rivinius was not surprised by the reaction of the astronomical community to their discovery of the black hole. “It is not only normal, but also necessary for results to be looked at critically,” he says. “Especially if it makes the headlines.”

This image shows the region of the sky where HR 6819 is located, a triple system consisting of two stars and the nearest black hole | © ESO – D. De Martin
Rivinius and his colleagues believed that the best explanation for their data – obtained with the MPG/ESO 2.2-meter telescope – was that HR 6819 is a triple star system, consisting of a star that orbits in 40 days. a black hole orbits and a second star in a much wider orbit around it. But research led by Julia Bodensteiner, then a doctoral student at KU Leuven, suggested a different explanation based on the same data.Pros: HR 6819 could also be a system without a black hole, consisting of only two stars orbiting each other in 40 days. This alternate scenario would require one of the stars to be ‘dismantled’, meaning it has lost much of its mass to the other star in the recent past.

“We had reached the limit of the existing data, so we had to apply a different observation strategy to investigate which of the two proposed scenarios is correct,” said KU Leuven researcher Abigail Frost, who led the new study.

To solve the research question, the two teams collaborated to obtain new, sharper data from HR 6819 using ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI) . “The VLTI was the only facility that could give us the data we needed to differentiate between the two explanations,” said Dietrich Baade, co-investigator on both projects.

Binary star without black hole
“The scenarios we were looking for were quite clear, very different and easy to distinguish with the right instrument,” says Rivinius, “We agreed that the system contains two light sources, so the question was whether they were close revolve around each other, as in the decommissioned star scenario, or are far apart, as in the black hole scenario.”

To differentiate between the two hypotheses, the astronomers used both the VLTI ‘s GRAVITY instrument and the Multi Unit Spectroscopic Explorer (MUSE) on ESO’s VLT.

“MUSE confirmed that there is no wider orbit bright companion, while GRAVITY’s high spatial resolution was able to distinguish two bright sources only 1/3 the Sun-Earth distance from each other,” says Frost. “These data proved to be the final piece of the puzzle, leading us to conclude that HR 6819 is a binary star with no black hole.”

Stellar Vampirism
“As far as we know now, we are dealing here with a star system in which one star has ‘sucked’ the atmosphere of its companion. This is a common phenomenon in compact binary stars, also known as ‘stellar vampirism’ ,” said Bodensteiner. “While the donor star lost some of its material, the receiving star has started spinning faster.”

“The chances of witnessing the aftermath of such an interaction are extremely slim because it lasts so short,” Frost added. “That’s what makes our conclusions about HR 6819 so interesting: it’s a perfect candidate to investigate how this ‘vampirism’ influences the evolution of massive stars, and associated phenomena such as gravitational waves and massive supernova explosions.”

This data proved to be the final piece of the puzzle, leading us to conclude that HR 6819 is a binary star with no black hole.

The new Leuven-ESO joint team now plans to take a closer look at HR 6819 using the VLTI’s GRAVITY tool. The researchers will jointly examine the star system to better understand its evolution, and use that knowledge to learn more about other binary star systems.

As for the search for black holes, the team remains optimistic. Stellar-mass black holes remain very elusive by their nature, Rivinius says. “But rough estimates suggest that there must be tens to hundreds of millions of black holes in our Milky Way galaxy alone,” Baade adds. It’s only a matter of time before astronomers discover them.

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